Project Summary/Abstract Drugs of abuse are known to cause long-term changes in the synaptic plasticity of reward circuitry and, ultimately, persistent changes in behavior. Understanding the mechanisms underlying these changes is a key open question. As the reward circuitry and the learning and memory circuitry share key nodal points, new discoveries in the learning and memory field will have the potential to shed light on aspects of drug-seeking behavior. In particular, it is well established that gene expression is required for long- lasting forms of synaptic plasticity and long-term memory formation. Very recently, studies demonstrated that epigenetic regulation of gene expression during memory consolidation can modulate gene expression dynamics, resulting in surprising effects on memory formation. For example, inhibition or deletion of histone deacetylase 3 (HDAC3) in the nucleus accumbens leads to maintained expression of Nr4a2 (nuclear orphan receptor 2), correlating with cocaine-context associated memory formation. Nr4a2 is a transcription factor for tyrosine hydroxylase and the dopamine transporter, defining enzymes of dopaminergic signaling. The medial habenula (MHb), a region that highly expresses both HDAC3 and NR4A2, and its projections have recently been identified as main targets of drugs of abuse. Recent work has shown that the MHb is implicated in both the acquisition and reinstatement of drug-seeking behavior. Furthermore, my preliminary data has shown that the MHb is active during the reinstatement of cocaine- associated memories. Yet, the effects that drugs of abuse have on the circuitry of the MHb and the epigenetic mechanisms that regulate its gene expression remain largely unstudied. The experiments in this proposal will address the overall hypothesis that the MHb is necessary and sufficient for reinstatement of cocaine-context memory (Specific Aim 1 and 2) in an Nr4a2-dependent manner (Specific Aim 3).